Method of producing monocrystalline semiconductor material



Jan. 1968 A. KERSTING ETAL METHOD OF PRODUCING MGNOCRYSTALLINE SEMICONDUCTOR MATERIAL- Filed Oct. 20, 1965 Fig.2

Fig.1

United States Patent Office 3,356,462 Patented Jan. 30, 1968 3,366,462 METHOD OF PRGDUGSKNG MONOCRYSTALLINE SEMECONDUQTOR MATERIAL Arno Kersting and Kiaus Wartenberg, Erlangen, Germany, assignors to Siemens-Schuckertwerke Aktiengesellschaft, Berlin, Germany, a corporation of Germany iled Oct. 20, 1965, Ser. No. 498,667 Claims priority, appiicafign Germany, Nov. 4, 1964,

9 .,023 1 Claim. (Ci. 23-301) Our invention relates to method for producing monocrystalline semiconductor material.

Semiconductor material of great purity in monocrystalline form is necessary for production of semiconductor devices such as rectifiers, transistors, photoelectric devices and the like. Many different methods are employed for manufacturing such semiconductor devices. In accordance with a known process, silicon or germanium is first obtained in the form of a polycrystalline rod by deposition from the gas phase on a rod-shaped carrier member. The deposition can be effected by thermal decomposition or by chemical reaction such as a reduction process. Thus, for example, silicon can be obtained from gaseous silicon compounds such as silicochloroform or silicon tetrachloride, for example, by the reduction thereof with hydrogen. The carrier member can have a diameter of 6 mm., for example, which can be increased by deposition to a diameter of 20 to 40 mm. or more.

Thereafter, the semiconductor rod thus obtained is subjected to a crucible free zone melting process wherein a monocrystalline seed crystal is fused to one end of the semiconductor rod, and the semiconductor rod is changed to a monocrystal beginning at the end to which the seed crystal is fused. Usually the semiconductor rod is not only changed to a monocrystal by the crucible-free zone melting process, but in addition is purified, and a uniform distribution of the dopant concentration over the rod cross section and over the rod length is ensured. The semiconductor monocrystalline rod thus obtained is subsequently divided into discs by slicing the rod perpendicularly to the longitudinal axis thereof, the discs being thereafter made into semiconductor structural elements or components by further processing. According to the particular purpose for which they are to be used, these discs require a specific crystal orientation which must be accordingly imposed upon the semiconductor rod beforehand. In most cases the Ill-axis is involved which must coincide with the longitudinal axis of the semiconductor rod whereby the sliced surfaces of the semiconductor discs are lll-surfaces. The alloying of metallic contact electrodes thereto can be especially promoted when the surfaces thereof to which the alloying is to be made are the Ill-surfaces. If necessary the ll-surface can also be used.

Another rocess of producing monocrystalline semiconductor material has become known, which consists of depositing monocrystalline semiconductor material from the gas phase on a monocrystalline rod-shaped carrier member. This material can also be divided by slicing the rod thus formed perpendicular to the longitudinal axis thereof into discs or wafers and thereafter further processing the wafers into semiconductor structural components.

For specific purposes, particularly for carrying out measurements, it has been found advisable to modify the aforedescribed known processes wherein semiconductor material is first deposited from the gas phase onto a rodshaped carrier body and thereafter transformed to a monocrystal bycrucible-free zone melting, by carrying out the deposition process so that the deposited semiconductor is originally in monocrystalline form. Thus, for example, during the deposition process measurement values regarding the quality of the semiconductor material can be obtained by interim measurements or readings at the monocrystal being formed, such measurements being unattainable for polycrystalline semiconductor material. With this modification of the known method, there arises, however, difiiculties relating especially to the further processing by crucible-free zone melting, because in most cases the monocrystal being formed has a cross section which renders further processing difiicult. Thus, for example, when employing a carrier member whose longitudinal axis coincides with the Ill-axis, the cross-sectional shape of the monocrystal being formed is that of a regular hexagon. The corners of this hexagon have a disturbing effect during the after-processing by crucible-free zone melting since it represents a very great deviation from the usual form of a circular cross section which arises as the result of crucible-free zone melting.

It is accordingly an object of our invention to avoid the foregoing disadvantages of the known process and particularly to provide a process for producing a rod of semiconductor material which does not have disturbing influences on subsequent processing by crucible-free zone melting.

With the foregoing and other objects in view, we therefore provide a method of producing monocrystalline semiconductor material, especially silicon, wherein semiconductor material is deposited from the gas phase onto a rod-shaped carrier member whereupon the semiconductor rod thus formed is changed into a monocrystal by a cruciblefree zone melting process employing a seed crystal. The semiconductor material is deposited in monocrystalline form on a monocrystalline carrier member whose longitudinal axis corresponds to the -axis and then the semiconductor rod is transformed into a monocrystal of anothe orientation.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method of producing monocrystalline semiconductor material, it is nevertheless not intended to be limited to the details shown, since various changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The method of this invention, together with additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a monocrystalline semiconductor rod of silicon, for example, produced by deposition from the gas phase, the rod longitudinal axis coinciding with the Ill-axis of the crystal;

FIG. 2 shows a cross-sectional view of a semiconductor rod produced by deposition out of the gas phase, wherein the longitudinal axis of the rod coincides with the 115-axis; and

FIG. 3 is an axial view of the rod shown in FIG. 1 as it is subjected to a crucible-free zone melting operation.

Referring now to the drawings, it is shown in FIG. 1 that a monocrystalline semiconductor rod produced by deposition on a carrier crystal oriented in the Ill-direction has the cross-sectional form of a regular hexagon. material deposited on a monocrystalline carrier rod On the other hand, FIG. 2 shows that the semiconductor oriented in the 1l5-direction has the shape of a duodecagon which is caused by the beveling of the respective corners of the hexagon of FIG. 1. Generally the duodecagon does not have the exact shape of a regular dudocegon.

FIG. 3 illustrates the transformation of the rod of hexagonal cross section as shown in FIG. 1 into a crystalline rod having a circular cross section as is effected with a crucible-free zone melting process. A hexagonal semiconductor rod 2 is shown in the upper part of FIG. 3. A rod portion 4, transformed by zone melting into a circular cross-sectional form, is separated from the hexagonal semiconductor rod 2 by the melting zone 3. The circular cross-sectional shape of the rod portion 4 is produced generally only by the effect of the surface forces acting on the melt 3. In most cases this effect is even further increased when an induction heating coil is employed as the heat source for the crucible-free zone melting process since due to its electromagnetic forces, the shaping of the melt and thereby of the material crystallizing out of the melt are promoted. Added to this, with a crucible-free zone melting process there is' generally provided a rotation of at least one of the rod holders (not shown) at the end of the rod, as indicated by the arrow A, whereby a further uniform distribution of the crosssectional shape to form a regular and uniform circle is achieved.

In the embodiment of FIG. 3, heating of the melting zone 3 is effected by an induction heating coil 5. As is seen from FIG. 3, difficulties tend to arise at the six corners of the hexagonal cross section of the rod 2 because they are not subjected to the heating effect to the same extent as the other more centrally located portion of the cross section. Consequently these corner portions of the hexagonal rod 2 will tend to melt somewhat later than the central portion thereof so that when the induction heating coil 5 moves with the melting zone 3 in an upward direction, portions of the edge of the semiconductor rod 2, namely portions at these corners thereof, melt belatedly and then run downwardly suddenly into the melt in the form of drops and thereby cause an undesired irregularity in the melting and freezing or hardening processes. This irregularity is naturally also produced with other heating means. It is thus possible that portions of these corners will contain seeds, which act as secondary seeds at the lower limiting or border surface of the melting Zone 3 and may thereby tend to disturb the crystal growth.

As is apparent from a comparison of the cross sections of the rods of FIGS. 1 and 2, the aforementioned danger of disturbed crystal growth is clearly avoided by the upper rod portion 2 being formed in the cross section shown in FIG. 2, so that in accordance with the features of the method of this invention, these difficulties are avoided.

The method of our invention combines several advantages. The semiconductor material is initially deposited in monocrystalline form on the monocrystalline carrier rod by uitably selected deposition conditions. The opportunity is consequently afforded during the deposition process or during pauses in the deposition process to carry out interim measurements by means of which the conductivity (specific resistance) or the type of conductance i.e. whether nor p-type, of the deposit material can be determined. Specific dopant additives are thereby, if desired, able to be controlled or adjusted so that a predetermined amount of doping and a predetermined type of dopant are produced over the entire cross section. The semiconductor material thereby grows in an orientation corresponding to the aforementioned orientation of the carrier rod. The semiconductor rod which is formed consequently assumes such a shape so that its cross section is that of a duodecagon. In the subsequent crucible-free zone melting operation, a circular cross section of the semiconductor rod is produced in a,

known manner due to the surface forces and due to the rotation, if desired, of the rod holder's. Such a circular form matches the cross-sectional shape of a duodecagon considerably more closely than other cross-sectional forms, such as a hexagon for example, which is produced at a 111-orientation of the longitudinal axis of the rod.

By means of the subsequent zone melting operation, the dopant concentration is distributed uniformly over the entire rod cross section and over the rod length and is reduced, if necessary, in a specific manner. The desired orientation of thesemiconductor rod is thus effected by the application of a suitable seed crystal. Provision can be made for having the longitudinal axis of the rod coincide with the 111-axis or with the llO-axis of the crystal lattice for example. No difficulties arise thereby because the semiconductor rod produced by deposition is not polycrystalline but rather monocrystalline with another orientation.

We claim:

1. Method for producing monocrystalline semiconductor material of the group consisting of germanium and silicon which comprises vapor-depositing the semiconductor material as a monocrystal on an elongated monocrystalline carrier member having a longitudinal axis corresponding to the 1l5-axis of the monocrystal; and subsequently subjecting the monocrystal to a cruciblefree zone melting operation for transforming the semiconductor material to a monocrystal having a longitudinal axis selected from the group consisting of the 110, and 111 axis.

References Cited UNITED STATES PATENTS 3,160,522 12/1964 Heywang 23223.5 3,160,521 12/1964 Ziegler 23-2235 NORMAN YUDKOFF, Primary Examiner.

G. P. HINES, Assistant Examiner. 

1. METHOD FOR PRODUCING MONOCRYSTALLINE SEMICONDUCTOR MATERIAL OF THE GROUP CONSISTING OF GERMANIUM AND SILICON WHICH COMPRISES VAPOR-DEPOSITING THE SEMICONDUCTOR MATERIAL AS A MONOCRYSTAL ON AN ELONGATED MONOCRYSTALLINE CARRIER MEMBER HAVING A LONGITUDINAL AXIS CORRESPONDING TO THE 155-AXIS OF THE MONOCRYSTAL; AND SUBSEQUENTLY SUBJECTING THE MONOCRYSTAL TO A CRUCIBLEFREE ZONE MELTING OPERATION FOR TRANSFORMING THE SEMICONDUCTOR MATERIAL TO A MONOCRYSTAL HAVING A LONGITUDINAL AXIS SELECTED FROM THE GROUP CONSISTING OF THE 110, AND 111 AXIS. 